If you are using a local mail server with postfix you may wish to look at some resources here from other users doing the same.
You may need to make sure you are pointing outside of the Bitwarden container to the host system where postfix is set-up, or make sure any docker networking is configured if in another container.
If at all possible I highly recommend trying to use an SMTP relay rather than hosting your own mail server, but hopefully these links help.